The present invention relates generally to a semiconductor fabricating apparatus having a load-lock chamber and, in particular, to a semiconductor fabricating apparatus which permits control of the oxygen concentration within a load-lock chamber, to a method for controlling the oxygen concentration within a load-lock chamber, and to a native oxide generating method in which the generation of native oxide on a wafer within a load-lock chamber is controlled by controlling the oxygen concentration within the load-lock chamber.
In a process for fabricating a semiconductor device by subjecting a silicon wafer to treatments such as thin film deposition and impurity scattering etching, unnecessary native oxide affects the product quality and yield substantially. Thus, a known semiconductor fabricating apparatus has a load-lock chamber of sealed construction, which is connected to a reaction furnace thereof, for accommodating treated hot substrates to thereby prevent the latter from being native oxidized.
Referring now to FIG. 5, description will be made as to the known apparatus having such a load-lock chamber.
Substrates to be treated are loaded on a cassette and then transported onto a semiconductor fabricating apparatus. The semiconductor fabricating apparatus includes a cassette stocker 2 disposed on a front side thereof, a reaction tube 3 disposed at a rear upper part thereof and a heater 4 disposed to extend around the reaction tube 3. The apparatus also includes a load-lock chamber 5 formed as an airtightly closed container, which is positioned at a lower side of the reaction tube 3 and connected to the reaction tube 3 by means of a gate valve 6. Inside the load-lock chamber 5, a boat elevator (not shown) is provided below the reaction tube 3. The boat elevator is capable of loading into and pulling out from the reaction tube 3 a boat 7 with wafers 10 loaded thereon. A gate valve 8 is provided on a side of the load-lock chamber 5 facing the cassette stocker 2. Between the cassette stocker 2 and load-lock chamber 5, there is provided a wafer transfer unit 9.
A wafer cassette 1 loading wafers 10 thereon is fed into the cassette stocker 2, where the wafers 10 are transferred through the gate valve 8 onto the boat 7 by means of the wafer transfer unit 9. The load-lock chamber 5, after vacuum withdrawing, is filled with an inert gas fed from a gas supply nozzle 11 to turn the inside thereof into an inert gas atmosphere. The gate valve 6 is then opened, and the boat 7 is inserted into the reaction tube 3. Upon completion of the treatment of the wafers 10, the boat 7 is pulled out into the load-lock chamber 5 and cooled down there to a predetermined temperature. Thereafter, the gate valve 8 is opened, and the wafers 10 on the boat 7 are transferred to the wafer cassette 1 of the cassette stocker 2 by means of the wafer transfer unit 9.
If oxygen is present in the atmosphere of the load-lock chamber 5 when the treated, hot wafers 10 are cooled therein, a native oxide layer is formed on the surfaces of the wafers 10 due to the native oxidization occurring on the surfaces, thus adversely affecting the properties of the resulting devices. Also, when the untreated wafers 10 loaded on the boat 7 are loaded into the reaction tube 3 of high-temperature atmosphere (approx. 650.degree.-750.degree. C.), oxygen within the atmosphere of the load-lock chamber 5 reacts with the surfaces of the wafers 10 under high temperature, and a native oxide (n-SiO.sub.2) layer forms on the wafer surfaces. The n-SiO.sub.2 layer is likely to grow in thickness more than expected, thus imparting adverse effects on the properties of the resulting devices. To suppress the generation of the native oxide layer, the inside of the load-lock chamber 5 is substituted by an inert gas, as described above.
Referring next to FIG. 4, description will be made as to a known apparatus for controlling the gas atmosphere within the load-lock chamber.
The gas supply nozzle 11 extending into the load-lock chamber 5 has a plurality of gas supply apertures (not shown) disposed at predetermined intervals along the length thereof so that the inert gas can be supplied in a diffused fashion. The gas supply nozzle 11 is connected to an inert gas supply source (not shown) and includes an air valve 12 and a flow rate adjuster 13 provided at an intermediate portion of a supply line of the gas supply nozzle 11. For achieving the balanced exhaustion of the atmosphere of the load-lock chamber, the load-lock chamber 5 has an inert gas exhaust pipe 14 communicating therewith at plural portions (only two shown in FIG. 4 for illustration), as well as an exhaust pipe 16 also communicating therewith. The inert gas exhaust pipe 14 has an air valve 15. The exhaust pipe 16 has an air valve 17 and a vacuum pump 18.
For turning the inside of the load-lock chamber 5 into an inert gas atmosphere by the known gas atmosphere control apparatus, the air valves 12 and 15 are opened, and an inert gas is then fed into the load-lock chamber 5 at a predetermined flow rate set by the flow rate adjuster 13 while exhausting the atmosphere of the load-lock chamber 5 through the inert gas exhaust pipe 14, whereby the atmosphere inside the load-lock chamber 5 is substituted by an inert gas atmosphere. In another instance, with the air valves 12 and 15 closed, the air valve 17 is opened and the atmosphere of the load-lock chamber 5 is exhausted once by means of the vacuum pump 18. Thereafter, the valve 17 is closed and the air valve 12 is opened to allow introduction of an inert gas in substitution for the atmosphere of the load-lock chamber 5. This achieves substitution of the atmosphere of the load-lock chamber 5 by the inert gas with oxygen concentration less than 1 ppm under an atmospheric pressure.
In each of the above-described instances, the atmospheric substitution is achieved to such a level that is almost the same as a complete inert gas atmosphere but control of the oxygen concentration is not possible. Depending on devices (e.g., at the time of generation of a gate dioxide (SiO.sub.2) layer on the MOS transistor of FIG. 2), it is preferable that a native oxide layer of appropriate level is formed on the surfaces of the untreated wafers 10 as the latter are loaded into the reaction tube 3 of hot atmosphere. This is found to be an n-SiO.sub.2 layer of the order of 1-2 atomic layers (less than 2 .ANG.) according to the data obtained through experiments conducted by the inventors hereof. It is not possible for the conventional apparatus to form such appropriate native oxide layer on the wafer surfaces.
In the light of these circumstances, the present invention enables to maintain the oxygen concentration in the internal atmosphere of a container at a desired value and to maintain the gas mixture concentration within a closed container at a desired value so that the generation of an n-SiO.sub.2 layer on wafers upon introduction of the wafer into a hot atmosphere can be controlled.